Hi, I have a 1973 Mercedes 450SL engine 117.982 chassis 107.044. A previous owner installed a Multi-Spark Discharge model MSD6A (I believe to replace the ballast resistors?) and now I want to upgrade to a Pertronix ignitor and coil. Pertronix does provide a wiring diagram which shows how to include the MSD6A but my question is: do I need the MSD6A any more at all? I mean the Pertronix ignitor wires directly to the coil so the MSD6A can be bypassed, right?

Pertronix is only a " trigger ", i.e. it replaces the ignition points.
The MSD 6A is a Capacitive Discharge System, and will increase the energy output, as well as provide Multiple Spark Discharge (" MSD ").
Go ahead and wire it up as per Pertronix's instructions.

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2007 C 230 Sport.

Thanks i'll try it that way.

Hey Todd,

I have a 74 450SL and did the points conversion from "original everything" to Pertronix points while keeping the stock coil (with no resistance) and a preresistor. The instructions were so simple that it scared me! But it worked perfectly. She runs like a top. I am anxious to see what king of gas mileage I get now. Used to be 11-12 mpg on 89 octane. As far as performance, I don't really notice an improvement, but perhaps hotter plugs and the high voltage coil will show more gittiup.

Well, almost. I had to give up my tach and cannot figure out how to pick the signal back up.

To run the Pert black and red wires to the coil (-) and 12+ respectively, that bypasses the Ignition Control Unit green wire that sends the tach its signal.

By looking at the schematic and simple deduction of what happens, the green wire wants to see the points close to ground to sense the RPM. BTW, to run the black wire to the coil (-), I had to remove the ground wire that was already on the post.

Where did you hook up your wires and did you have to also pull the coil ground? Did you have any other trouble in doing so??

I'm at the end of the road until I can find some help here.

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Scott
1974 450 SL (214K)

The signal should be the same to the coil. You just have a new set of points. If you connect the tack and it doesn't work call the new point company for help.

You are right, Gary. It seems simple and I will try tech support tomorrow again and see if I get a different guy that knows what to do here.

The issue I see is this: The "new" points have not just a single wire and a chasiss ground that bolts to the dizzy plate and therefore to ground. The old points originally open and close the connection to ground right? That should power up the coil for each plug firing and also the ICU sees that sequence and timing to create a tach signal. And that single points wire went to the green tach sense connection. Makes sense to me.

Well, the Pertronix unit has 2 wires and and bolts to the dizzy plate (electrical ground.) The red wire that must see 12v+ and no less or it won't trigger. To fire the coil up and therefore get ignition when cranking the starter, the black wire must connect to the coil (-). That coil post had a ground wire on it, which I had to remove before hooking up the Pert black to make her run.

I have tried the obvious way - just connect Pert black to the terminal where the old points wire went (to the green tach sense terminal!) and the red to 12v+, but no joy. In fact, that was per the first Pertronix tech guy guessing at how to get it right, but it just cranked - no firing at all.

I tried a couple other possible ways (quickly with jumpers) to see if it would even kick on crank, but got no firing at all.

So I am left lost in the jungle here. Maybe if I run into Sawyer... or better yet, Freckles....

Anyone with ideas, I would love to try them.

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Scott
1974 450 SL (214K)

I'll try an explanation. This is from a my knowledge of how the ignition coil works.

The points actually ground the coil's internal two coils through (or at) the - terminal. Both the primary (12V) coil and the secondary (12 -20,000V)coils go to ground through the same - (neg.) terminal. So the fire (in my eyes), is the +12V on the + term of the coil. More about that later w/ballast resistors.

To send a spark to the dist, The rotor in the distributor 'points' to a plug wire as the spark is coming out of the secondary coil. To make the spark

(Sorry, but that's what it does. I wouldn't call it a switch because it does not close like a switch. And Sorry the points close like a switch and they are not pointed or pointing at anything!)

the coil -term is grounded.

The 12V primary gets a path to ground and quickly builds a magnetic 'field' in the core inside the two coils. This sucks amps from the +12 supply through the coil and to ground though the points.

The field stays for a "dwell" time. Then the points open again and the mag field colapses and pow: the field cutting through the seconday coil generates a voltage in the secondary winding. The secondary has about 30 times more windings (you've seen like 30:1 on coils?) in the coil so the "resultant" secondary voltage is much bigger (can't remember the math) and much lower current.

The problem with points was/is that as the points open and close with voltage and curent on them, a little spark is pulled. A bigger one happens when they start to open. The voltage starts reversing, going to zero. That's what the capacitor was for: it dampens the arc and makes the points last longer. The capacitor conducts only when voltage is changing. They pass AC and block DC. The pitting comes from metal molecules moving with the spark.

Where was I? Sorry. A simple electronic ignition from the 60's left the points in and used them to turn on high current transistors that put the ground to the coil. This eliminated the high current jumping accross the points making them last indefinately. I've still got 1970 points in my 67 250SL because of an added CD ignition. That's a later topic, we aren't done with the points.

The moduals that go in the dist to replace the points, is typically a transistor device like the old school 'transitor ignitions' that substute for the points. They will have a trigger device that drives or signals the transistor circuit to 'close'. The transistor is just like the points except they need a voltage (12V coming through the coil to the - terminal. If you measure the voltage at the -term with points open and ignition on, you see 12V between it and ground. The transistor pulls that 12V down to almost ground (zero volts) faster than the points could. That's why the old cam disapeared replaced by faster signaling devices to hit the coil faster and let it colapse faster. The voltage rings higher. I forget the math, has to do with frequency and time.

When you put a scope on the -terminal, (engine running), you see the signal (voltage) going from about 12 down to below 0 and the voltage 'rings' up and down passed these points (sorry again) as the coil is closed to ground and opened again.

This describes the 'primary scope circuit patern.' Putting the scope on the secondary shows the result on the secondary or high voltage side.

On multi coil new school systems, now that we have a computer, the computer sends a signal to a coil on each spark plug (I think one coil is used on dual spark plug, but its grounded in the middle of a bigger secondary resulting in one signal,one coil and two secondies for the plugs). This is as electronic as it gets (for now) and it is really computer controlled electronic spark control. An engineer can custom design the spark for specific conditions.

Ballast resistors? They are a compromise and only needed because the coil is designed to give the best (hottest, biggest, highest voltage) spark when starting. Its also cold. As the coil gets hit with 12V, many amps, at 5000 times a second, it warms up. How warm it gets is a measure of inefficiency: some of the voltage and current are turned into heat. Most important it reduces life. By putting a heavy duty (can disapate watts of power without melting like a hot plate) resistor between the +12 and + term on coil, the voltage and current (power) available to the coil is reduced to where the spark is good enough and the coil stays cool enough to last for lots of years of use.

Ballast resistors aren't needed if the voltage to the + terminal is computer controlled. Old ignitions have a wire coming from the starter that by-passes the resistor and puts 12V right to the coil while the starter is on. Its the little terminal on the starter sellonoid.

Add-on aftermarket ignitions that replace points, do some of the above. Some just replace the points, some can change the signal to the coil based on what ever they are setup to measure, current, voltage, frequency (rpm), vacuum... They can adjust the voltage between start and run, eliminating the ballast resistor. To do that they might have a wire that replaces the +12V wire from the starter and the ballast wire or it could keep the starter wire on for starting and control the 12V through its wire once the engine starts.

So now you should have a good idea of what these two moduals do and what they need. one just replaces the points and has a third wire to 12V to power is circuits: 12V supply and ground for itself and a wire going to the coil -term. It also gets a signal from the dist when and how long to short the coil to ground.

The other module can do the point thing or use the point signal from the three wire device. This second module probably does some signal conditioning to control how the coil is shorted, opened, duration and speed of signal change, maybe control the 12V to the +term.

The only problem is how much current your tack circuit needs t o trigger the tack curcuit. If it is high impedence it will not draw enough current to change how the transistor shorts the coil to ground. (and it should be high. Impedence is like resistance but changes with frequency, in this case we expect the resistance to get lower as the rpm goes up so it migh work at idle but loose signal or confuse the signal as revs go up.)

I suppose you could measure it with an ohm meter and get a place to start. It should be high resistance maybe minimum 10 to 20k ohms would be ok. That is comparable to a very old cheap voltmeter. Connecting it will not stop the motor or change the primary signal patern as you watch it on a scope. A scope is very high impedence (meg ohms?) so not to effect the circuit you want to watch.


This is probably way too much detail for a new guy so I'll finish.

So look at how these things connect to the coil and see if you can figure out what they are doing, where the power comes from, where the grounds are, and what they do to the coil.

Then ask more questions

Gary, thank you for the specifics on the coil/points/ballR system! That was very informative. As an amateur radio enthusiast, it reminded me that even though I made it to Extra class, I have forgotten a lot.

This project is probably quite simple, but then I have been told (by my better half and coworkers) that I over analyze things. Still, I just feel impelled to understand things completely. Thanks again. Your reply her emphasizes the knowledge base that exists here in these forums.

They/you are INVALUBLE!

I'm gonna print this out and digest it tonight and tomorrow and then see if I can phrase my issue and see if you agree or otherwise point me in a direction. Work beckons tomorrow, too. Hurricane force storms off Oregon and the 10 tug/barge units that I deal with will need careful attention.

I'll advise on the existing wires and conditions that I can identify. I've already been checking them, but it's a little late to try to spit it all up. I'm sure I would confuse.

Like the little red-head said - Tomorrow, tomorrow...
Scott

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Scott
1974 450 SL (214K)

Your early107 has the First Version Transistorized ignition system [ There were 3 versions]
The oddity of this first version was the fact that they factory grounded the coil to chassis ground and the ECU switched the coil from the POSITIVE side of the circuit . This is directly opposite to conventional point systems where they operate on a Switched Ground circuit [ the points simply ground the neg terminal/side of the coil, thus completeing the circuit.]
This simply means the coils switching/on-off of the circuit is done BEFORE the coil , whereas, the standard point system switches/grounds the circuit AFTER the coil. Which is why that little pigtail ground wire in the distributor is so important for anything to work.
The Pertronix unit does this exact same switching of the ground side of the coil, but rather than doing it with a set of mechanical points, it does it with a switching transistor..that is all the unit does ..it grounds the black neg wire from the coil , just like the points did.
SO, when one uses a Pert on these early trans types, all the ECU circuit has to be eliminated and the Pert is wired directly to the coil with the original factory circuit ground wire on the coils neg side eliminated.
The problem now is the tach needs a trigger sig..I think you can do that on the coils neg side , but you may have to still use the other ECU resistor [ .04] of the original unit to get the tach to be accurate, as that resistor was a dropping device for the ECU tach circuit. that may explain your high tach readings]. The other [ .06] was the actual ballast resistor.
I have that exact info here somewhere and will look for it.

When I did my conversion I couldnt see how it would work with the original coil as one side of the secondary winding needs to be permanently grounded to make a circuit with the plugs so I replaced the coil as well with a conventional one.

Attached Thumbnails

 

That is the Perts recommendation..they want a coil circuit of approx 3 ohms for max protection of their module.
The Pert11 has the protection for overdraw, but in doing so they lost the simplicity of the Pert1 circuitry. Many guys just don't care for the later version, so they go with the Pert1 and the Pert Flamethrower coil, as they are a match..
I prefer Crane Optical as start-up spark is strong at low rpm, whereas induction pickups are weak at low crank speeds by there nature of voltage rise with reluctor speed. Also much cleaner on/off noise circuit w/optical switching.
..but they both are miles better than mechanical switching, which went out with horse shoes...even you weed-wacker has electronic ignition............

OK. So! I am the lucky owner of the Other Kind of breaker points ignition - the positive-switched, not ground-switched kind. Does this mean that the Pert is a ground switching unit and my system isn't so I cannot use it here if I want my tach to read right?

I think I already tried to hook the Pert black wire to the Green/Yellow wire junction (connection C116 in the attached diagram) which is where the points were connected. To go that way, the coil (-) had to go to ground to complete that part of the circuit - just as in the diagram. But that didn't get any ignition on cranking. Maybe I didn't have it that want and should try it again...

With this system and the schematic I've attached, what do you think should work and what tach signalling issues might be there?

As a reminder, the only way so far that it hooked up and ran was with 12+ on pert red, pert black going to coil (-) and the coil ground wire removed. She runs perfect that way, too!! To get the tach to move I found that a jumper going from the Green/Yellow (C116 junction) to the coil (-) made it read about double and very unstable.

Thanks again all for the guidance here. It's an interesting puzzle.

Attached Files

Ignition.pdf (45.9 KB, 212 views)

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Scott
1974 450 SL (214K)

Well, after connecting the wires per the third Pert tech support guy (black to the green tach wire from the ICU and the red to 12v+), it works fine! Tach and all. While I thought I had originally tried it that way, I must have been mistaken. Too bad this isn't the first time it's gone this way...

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Scott
1974 450 SL (214K)